Field effect devices, such as field effect transistors, are fundamental components in modern electronics. They are basic components in most digital and many analog circuits, including circuits for data processing and telecommunications. Indeed it has been surmised that field effect transistors are among the most numerous of human-made objects.
Field effect devices typically comprise a controllable-conductivity path, called a channel, disposed between a source and a drain. A gate electrode is formed on a thin dielectric film overlying the channel. For example, the source and the drain can be n-type regions of silicon and the channel can be a p-type region connecting them. The gate electrode can be a conductively-doped polysilicon layer formed on a thin layer of silicon oxide overlying the channel.
If no voltage is applied to the gate, current cannot flow from the source to the channel or from the channel to the drain. However if a sufficient positive voltage is applied to the gate, electrons are induced into the channel region, thereby creating a continuous n-type conductive path between the source and the drain.
Capacitors are also important components of integrated circuits. A typical capacitor comprises first and second conductive layers separated by a thin dielectric film. In many circuits, such as memory circuits, capacitors and field effect devices work in conjunction and are formed from common layers.
The reliable operation of the integrated circuits is critically dependent on the reliability of the increasingly thin dielectric layers used in the circuit devices. As transistors have become smaller and more densely packed, the dielectrics have become thinner. Capacitor and gate dielectrics are now often less than 80 angstroms in thickness. With the arrival of ULSI technology, gate dielectrics are approaching thicknesses of 50 angstroms or less. For integrated circuits to work, these thin layers in each of thousands of different transistors must insulate the gate, protect the channel from migration of impurities, and resist damage from current. These demanding requirements may soon exceed the capacities of conventional silicon oxide layers.
Nitrogen doping has been used to enhance the reliability of silicon oxide dielectrics. However, in films so thin as those desired for use in VLSI and ULSI integrated circuits, it has been difficult to control the distribution of nitrogen in the film. As a consequence, it has not been possible to provide a nitrogen concentration profile that will simultaneously minimize impurity diffusion and current damage. Accordingly there is a need for devices with improved dielectric layers and methods for making them.